Field of the Invention
The present invention relates to an exhaust valve spindle for an exhaust valve in an internal combustion engine, which exhaust valve spindle has a shaft and a valve disc at the lower end of the shaft, which valve disc at its upper surface has a seat area, wherein the valve disc is of a base material and the seat area is of a seat material different from the base material.
Background
EP-A-0 521 821 describes a valve made of Nimonic 80A or Nimonic 81, which is provided with a layer of Inconel 625 or of Inconel 671 in the seat area to impart to the seat a higher corrosion resistance than the Nimonic base body. The publication mentions for the alloy Inconel 671 that it only has to be welded on, while for the alloy Inconel 625 it mentions that after the welding it contains a dendritic carbide structure and that the seat area therefore has to be hot-worked to homogenise the carbide distribution in the structure in order to improve corrosion resistance. Inconel alloy 625 is mentioned to have a composition of up to 0.10% C, up to 0.5% Si, up to 5% Fe, up to 0.5% Mn, 21% Cr, up to 0.4% Ti, up to 0.4% Al, up to 1.0% Co, 9% Mo, 3.6% Nb+Ta, and the balance Ni. Inconel alloy 671 is mentioned to have a composition of 0.04 to 0.05% C, 47 to 49% Cr, 0.3 to 0.40% Ti, and the balance Ni. EP-A-0 521 821 describes the finding that Inconel 671 having about 48% Cr does not provide a corrosion resistance as good as that of Inconel 625 having 21% Cr. Nimonic and Inconel are proprietary trademarks of Special Metals Inc.
The book ‘Diesel engine combustion chamber materials for heavy fuel operation’ published in 1990 by The Institute of Marine Engineers, London, collected the experience gained from the whole industry and in the resulting chapter on pages 131-143 conclusions were made. In a valve having a seat material different from the base material, the book calls the seat material a hardfacing. The hardfacings standing out during all the tests were Deloro alloy 60: 16% Cr, 4.5% Si, 4.5% Fe, 3.5% B, 0.5% C, and the balance Ni; Colmonoy 8: 26% Cr, 4% Si, 3.3% B, 1.0% Fe, 0.95% C, and the balance Ni; and Tristelle TS2: 35% Cr, 12% Co, 10% Ni, 5% Si, 2.0% C, and the balance Fe. It was concluded on page 143 that chromising offers no significant improvement in corrosion protection to nickel based alloys for exhaust valves. It is quite clearly difficult to choose an alloy having the required properties for a seat material in an exhaust valve, in particular when the fuel burned in the engine contains vanadium and sulphur.
WO 96/18747 describes an exhaust valve spindle with a welded-on hardfacing alloy analyzed at 40-51% Cr, from 0 to 0.1% C, less than 1.0% Si, from 0 to 5.0% Mn, less than 1.0% Mo, from 0.05 to 0.5% B, from 0 to 1.0% Al, from 0 to 1.5% Ti, from 0 to 0.2% Zr, from 0.5 to 3.0% Nb, an aggregate content of Co and Fe of 5.0% at the most, at the most 0.2% O, at the most 0.3% N, and the balance Ni. Following the welding a high hardness of 550 HV20 may be imparted to the seat material by means of a heat treatment at a temperature exceeding 550° C. It is generally presumed that hot-corrosion-resistant alloys containing chromium and nickel, age-harden at temperatures ranging from 550° C. to 850° C., with the result that the alloy becomes harder and more brittle. Attempts to make welded-on hardfacings having a high content of Cr are typically damaged by hot cracks developed during solidification and cooling at the welding process, however WO 96/18747 describes that the content of B of at least 0.05% (preferably at least 0.2% boron) in the alloy causes the hot cracks to be reduced or disappear. Furthermore, the content of Ti should be kept below 0.1% and the content of Al should be kept below 0.1% in order to suppress precipitation hardening at welding. WO 96/18747 discloses three specific alloys, all having above 48% Cr and 0.1% B, and in these alloys the precipitation hardening mechanism acts so slowly that the alloy substantially does not harden at welding onto the valve, but hardens during a subsequent heat treatment.
In the case of cast members, to achieve excellent hot corrosion resistance, particularly in environments containing sulphur and vanadium from heavy fuel oil combustion products, it is known to use an alloy of the type 50% Cr and 50% Ni, or an alloy of the type IN 657 having a design composition of 48-52% Cr, 1.4-1.7% Nb, at the most 0.1% C, at the most 0.16% Ti, at the most 0.2% C+N, at the most 0.5% Si, at the most 1.0% Fe, at the most 0.3% Mg, and the balance Ni. After casting, the alloy comprises a nickel-rich γ-phase and a chromium-rich α-phase where both phases, depending on the accurate analysis of the alloy, may constitute the primary dendrite structure. It is known that these alloys age-harden at operating temperatures exceeding 600° C. As the alloy after casting cools off, the alloy does not solidify in its equilibrium state. When subsequently the alloy is at operating temperature, precipitation of the under-represented phase proportion occurs by transformation of the over-represented phase proportion, which causes embrittlement characterized in a ductility of less than 4% at room temperature.
The exhaust valve spindle can be provided with a corrosion resistant alloy on the lower surface facing the combustion chamber. This lower surface is far from being subjected to mechanical action, like the influences acting on the valve seat, but the corrosion resistance needs to be high. WO 97/47862 generally describes an alloy comprising from 38 to 75% Cr, from 0 to 3.0% Nb, and a balance of Ni, where the corrosion-resistant alloy has a hardness of less than 310 HV measured at approximately 20° C. after the material has been heated to a temperature within the range of 550-850° C. for more than 400 hours. A specific example described is an alloy comprising 46% Cr, 0.4% Ti, 0.05% C, and the balance Ni.
From the market it is known to provide a welded-on hardfacing of Inconel 718 onto a valve base material of stainless steel where the hardfacing is rolled and heat treated in order to provide the seat material with the desired mechanical properties. In Inconel 718 the content of Fe is necessary for the slow precipitation of strength increasing particles. This slow precipitation is considered unique to Inconel 718 and has provided this alloy with special advantages among superalloys within in particular the turbine industry. Inconel 718 has the composition 19% Cr, 5.2% Nb, 0.9% Ti, 52.5% Ni, 3% Mo, 0.5% Al, and the balance Fe. A valve of this type has the advantage that Inconel 718 has excellent mechanical properties as required for a valve seat area, including strength and ductility. Unfortunately, a valve of this type does not have a corrosion resistance at high temperatures comparable to or better than the corrosion resistance of Nimonic 80A. Nimonic 80A is not weldable, and consequently it is required to make the entire valve disc out of Nimonic 80A as it cannot be provided as a hardfacing on a base material like stainless steel. Nimonic 80A has the composition 19% Cr, 2.4% Ti, 0.2% Fe, 1.7% Al, and the balance Ni. It is of course an advantage to use an iron-based base material instead of a nickel-based base material.